1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to an element isolating structure of the semiconductor device.
2. Description of the Background Art
In a semiconductor device, individual semiconductor elements are electrically isolated by LOCOS isolation or trench isolation. In the case where an integration degree of the semiconductor device is relatively low, the LOCOS isolation is enough. However, the trench isolation has been increasingly required with an enhancement in the integration degree.
An example of a structure of the trench isolation will be described with reference to FIG. 25. In FIG. 25, a plurality of MOS transistors are formed on a P-type silicon substrate 61, a trench is provided on the silicon substrate 61 between the MOS transistors in order to electrically isolate the individual MOS transistors, and a silicon oxide 66 formed by CVD (chemical vapor deposition) is buried in the trench to form a trench portion 65.
An N-type source-drain layer 62 constituting the MOS transistor is provided in a surface of the silicon substrate 61 on both sides of the trench portion 65, a gate oxide film 63 is provided to cover the trench portion 65 and the source-drain layer 62, and a gate electrode 64 is provided from the source-drain layer 62 to a channel region.
During an operation of the MOS transistor, a depletion layer covering the source-drain layer 62 also extends to the trench portion 65 side as well as the channel region side. By the existence of the trench portion 65, it is possible to prevent a punch through from being generated between the source-drain layers 62 on both sides of the trench portion 65.
However, when the microfabrication of the semiconductor device is improved to obtain a design rule of 0.15 xcexcm or less, a spacing between elements is reduced and the depletion layers extending from the source-drain layers 62 on both sides of the trench portion 65 shown in FIG. 25 approach each other beyond a bottom of the trench portion 65 so that a punch-through is generated between the source-drain layers 62 to easily cause a current leakage.
In order to avoid such a situation, a depth of the trench should be increased to inhibit the depletion layers from coming in contact with each other and a P-type diffusion layer having a high concentration should be provided on the outer periphery of the bottom surface of the trench portion to suppress the extension of the depletion layers. However, if the depth of the trench is increased, there is a possibility that an insulator might be buried therein with difficulty or a crystal defect might be generated in the silicon substrate due to a stress generated with the formation of the trench.
Moreover, in the case where the P-type diffusion layer is provided on the outer periphery of the bottom surface of the trench portion, an electric field in the depletion layer is increased depending on the concentration of a P-type impurity so that a junction leakage is increased by an electron trap assist tunneling phenomenon in which carriers are excited to a conduction band through a defect state in some cases.
In the structure described with reference to FIG. 25, furthermore, a positive charge is induced into the silicon oxide 66 and a negative charge is induced into the silicon substrate 61 in the vicinity of an interface between the silicon oxide 66 of the trench portion 65 and the silicon substrate 61 and a channel (a so-called side channel) using the silicon oxide 66 as a gate oxide film is generated to form a parasitic MOS transistor, resulting in the generation of a leakage current.
In the MOS transistor formed on the P-type silicon substrate, there has been known the fact that a leakage current generated between the source-drain layers with a gate voltage set to a ground level is decreased when a threshold voltage is increased. In order to raise the threshold voltage, it is preferable that a substance having a great electron affinity should be used as a gate material. This is the same as in the above-mentioned parasitic MOS transistor. By burying the substance having a great electron affinity in the silicon oxide 66 of the trench portion 65, a threshold voltage of the parasitic MOS transistor can be raised to reduce a leakage current.
As means for preventing the leakage current from being caused by a punch-through, a structure in which an electric conductor is buried in a trench portion has been proposed in addition to the above-mentioned means.
An example of a structure implementing the preventing means is illustrated in FIG. 26. Such a structure has been disclosed in Japanese Patent Application Laid-Open No. P01-138730A (1989), for example. In FIG. 26, a trench portion 55 is provided in place of the trench portion 65 shown in FIG. 25. The trench portion 55 is constituted by a silicon oxide film 56 provided on an inner surface of a trench, a compensating material layer 57 provided on an inner surface of the silicon oxide film 56, and an insulator 58 provided in a space defined by the compensating material layer 57. Other structures are the same as those in FIG. 25.
The compensating material layer 57 serves to compensate for a negative charge in a silicon substrate 61. As the compensating material layer 57 are used a substance having a great electron affinity, for example, a polysilicon layer doped with boron (B), aluminum (Al) or the like, a silicide layer such as a titanium silicide (TiSi) layer, a tungsten silicide (WSi) layer or the like, or a layer made of a refractory metal such as titanium (Ti), molybdenum (Mo) or the like.
In the structure shown in FIG. 26, however, there is a problem in that the compensating material layer 57 in the trench portion 55 is kept in a floating state, the amount of electric charges present in the compensating material layer 57 is varied depending on a manufacturing situation and a leakage current is reduced with difficulty.
The compensating material layer 57 is introduced to act as a substance to have a small work function difference between the silicon substrate 61 and the insulator 58, in other words, serves to change the characteristics of a material to fill in the trench. Accordingly, the compensating material layer 57 should be used in the floating state. However, the floating state sometimes causes electric charges to be stored in the process of manufacturing the semiconductor device such as ion implantation. Moreover, it is also supposed that the amount of electric charges is not constant and might cause a leakage current to be generated.
FIG. 27 also illustrates a structure in which an electric conductor is buried in a trench portion. The structure shown in FIG. 27 has been disclosed in Japanese Patent Application Laid-Open No. P08-172124A (1996), for example. An insulating film 77 is provided on an inner wall surface of a trench 72 formed in a semiconductor substrate 71, and a conductive film 78 is provided on an inner wall surface of the insulating film 77 and a bottom of the trench 72 and is in contact with the semiconductor substrate 71 at the bottom.
Moreover, an insulating film 79 is buried in a space defined by the conductive film 78, and an insulating film 74 is provided over the trench 72 to protrude therefrom.
In the structure shown in FIG. 27, if an electric potential of the conductive film 78 is to be fixed, it is necessary to control the electric potential of the conductive film 78 according to an electric potential of an N-type semiconductor region (not shown) present on both sides of the trench 72. However, it is hard to control the electric potential of the conductive film 78.
More specifically, in the case where an electric potential of the semiconductor substrate 71 is set to 0 V, a current does not flow between the conductive film 78 and the semiconductor substrate 71 if the electric potential of the conductive film 78 is also set to 0 V. However, in the case where the electric potential of the N-type semiconductor region present on both sides of the trench 72 is not 0V, a current flows between the conductive film 78 and the N-type semiconductor region.
Accordingly, the electric potential of the conductive film 78 should be controlled in consideration of the electric potential of the N-type semiconductor region present on both sides of the trench 72 and the electric potential of the semiconductor substrate 71. Thus, it is predicted that the control would be performed with difficulty.
Similarly, the structure in which a conductive film is provided in contact with a semiconductor substrate on a bottom of a trench has also been disclosed in Japanese Patent Application Laid-Open Nos. P06-140500A (1994) and P63-122145A (1988) which have the above-mentioned problems.
In order to solve the above-mentioned problems, it is an object of the present invention to provide a semiconductor device in which an electric potential of an electric conductor is controlled to reduce both a leakage caused by a punch-through and a junction leakage in a trench isolating structure having the electric conductor in a trench portion.
A first aspect of the present invention is directed to a semiconductor device comprising a semiconductor element formed on a semiconductor substrate and having a source-drain layer, and a trench isolating structure for electrically isolating the semiconductor element adjacently to the source-drain layer, the trench isolating structure including a trench provided in a surface of the semiconductor substrate, an electric conductor provided in the trench and having an uppermost portion in a position which is deeper than the deepest portion of the source-drain layer, an insulating film provided between a side surface of the electric conductor and the trench, and an insulator to fill in the trench on an upper portion of the electric conductor.
A second aspect of the present invention is directed to the semiconductor device, wherein the insulating film also extends between a lower main surface of the electric conductor and the semiconductor substrate.
A third aspect of the present invention is directed to the semiconductor device, further comprising a charge storage electrode, and a control system for automatically controlling an electric potential of the electric conductor, the control system comprising a first circuit portion including a first transistor of a first conductivity type having a first electrode connected to a first power source, a first resistive element having a first end connected to a second electrode of the first transistor, and a second resistive element having a first end connected to a second end of the first resistive element and a second end connected to a second power source for supplying an electric potential having a reverse polarity to the first power source, and a second circuit portion including a second transistor of a second conductivity type having a first electrode connected to the second power source, a third resistive element having a first end connected to a second electrode of the second transistor, and a fourth resistive element having a first end connected to a second end of the third resistive element and a second end grounded, wherein a control electrode of the first transistor is connected to the source-drain layer connected to the charge storage electrode, a control electrode of the second transistor is connected to the second end of the first resistive element, and the second end of the third resistive element is connected to the electric conductor.
According to the first aspect of the present invention, the amount of electric charges on a surface of the trench isolating structure can be controlled by giving a predetermined potential to the electric conductor. Therefore, a depletion layer extending from the source-drain layer of the semiconductor element isolated by the trench isolating structure can be prevented from being conducted through the periphery of the trench isolating structure to cause a punch-through state. Consequently, the generation of a current leakage can be reduced. The uppermost portion of the electric conductor is provided in the position which is deeper than the deepest portion of the source-drain layer. Therefore, an insulation between the electric conductor and the source-drain layer is improved. Thus, the controllability of the amount of the electric charges on the surface of the trench isolating structure can be enhanced by reducing a thickness of the insulating film between the side surface of the electric conductor and the trench.
According to the second aspect of the present invention, the insulating film also extends between the lower main surface of the electric conductor and the semiconductor substrate. Therefore, the electric conductor is insulated from the semiconductor substrate. When determining an electric potential of the electric conductor, it is not necessary to take an electric potential of the semiconductor substrate into consideration. Thus, the electric potential of the electric conductor can be set easily.
According to the third aspect of the present invention, in the case where electric charges are stored in the charge storage electrode, resulting in the necessity of preventing a leakage current, the electric potential of the electric conductor is automatically controlled based on an electric potential of the source-drain layer connected to the charge storage electrode. Therefore, it is possible to prevent a punch-through from being caused between the elements isolated by the trench isolating structure, thereby automatically reducing the generation of a current leakage.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.